Exposure method and apparatus

ABSTRACT

A method of exposing a photosensitive substrate with a band-narrowed laser beam from an excimer laser includes providing a stop having an opening in a path of the laser beam; adjusting a size of the opening of the stop to substantially compensate for a change in bandwidth of the laser beam; and exposing after the adjustment the substrate with the laser beam.

This application is a continuation of application Ser. No. 08/092,568filed Jul. 16, 1993, which is a continuation of application Ser. No.07/666,127, filed Mar. 7, 1991, both now abandoned.

FIELD OF THE INVENTION AND RELATED ART

This invention relates to an exposure method and apparatus. Moreparticularly, the invention is concerned with an exposure method andapparatus suitably usable for manufacture of semiconductor devicesthrough exposure using a laser beam of narrowed bandwidth.

In order to meet the recent requirement of a further increase in thedegree of integration of semiconductor devices, a circuit pattern of alinewidth of about 0.5 micron has to be transferred onto a wafer. As anexposure apparatus that enables transfer of such a very fine circuitpattern, an example is a projection exposure apparatus that uses areduction projection lens system and a laser beam of a KrF excimer laserof a wavelength 248.4 nm, for projecting an image of a circuit patternof a reticle upon a wafer.

Such a projection exposure apparatus is disclosed in British Patent No.2,153,543 and U.S. Pat. Nos. 4,811,055, 4,711,568, 4,905,041, 4,974,919,4,968,868 and 4,773,750.

The aforementioned U.S. Pat. No. 4,773,750 shows a projection exposureapparatus wherein the bandwidth of a laser beam from an excimer laser isnarrowed to about 0.005 nm, in order to suppress chromatic aberration tobe produced by a projection lens system. Generally, such bandwidthnarrowing may be effected by providing an etalon, a prism, a diffractiongrating or the like in a resonator of the laser.

FIG. 1 shows an example of an excimer laser wherein a prism and adiffraction grating are provided in a resonator. Denoted in the drawingat 1 is a laser; at 2 is a reflection type relief grating disposed at anend of a beam path in the resonator; at 3 is a prism having a functionfor expanding the beam diameter; at 4 and 4' are stops each having anaperture; at 5 and 5' are windows of a discharging chamber 6; at 7 isone of a pair of discharging electrodes disposed opposed to each otherin a direction perpendicular to the sheet of the drawing; at 8 is anoutput mirror; and at A is a laser light emanating from the outputmirror 8.

When an electric voltage is applied to the discharging electrodes 7 inthe discharging chamber 6, an excimer gas contained in the dischargingchamber 6 is excited and, from the resonator formed between the outputmirror 8 and the grating 2, a band-narrowed laser light is emitted.Here, the spectral width of the laser light influenced by the grating 2can be analyzed as follows:

If the pitch of lines of the grating 2 extending perpendicularly to thesheet of the drawing is denoted by d, the angle of incidence of thelight upon the grating 2 is denoted by θ_(B) and the order of reflectivediffraction at the grating 2 is denoted by m, then the wavelength λ tobe selected by the grating 2 can be given by the following equation:

    λ=(2d/m)sinθ.sub.B                            ( 1)

Since the light incident on the grating 2 has an expansion of an angleΔθ_(B), from equation (1), the spectral width Δλ_(B) of the lightreflectively diffracted by the grating 2 back to the prism 3 isexpressed as follows:

    Δλ.sub.B =(2d/m)cosθ.sub.B ·Δθ.sub.B( 2)

Here, if light having no expansion such as a plane wave is inputted tothe grating 2, the spectral width Δλ_(K) of the light coming back to theprism 3 can be expressed as follows:

    Δλ.sub.K =(d/mD)                              (3)

wherein D is the width of a region, to be illuminated, in a directionperpendicular to the grating line of the grating 2. Generally, there isa relationship of Δλ_(K) <Δλ_(B).

An excimer laser or a laser having a similar resonator structure, has anexpansion of spectrum as represented by equation (1). For this reason,irrespective of the fact that a band-narrowing element such as anetalon, a prism, a diffraction grating or the like is provided in theresonator, there is a tendency of relatively expanding bandwidth(halfwidth: FWHM) of a laser light emanating therefrom. If the intensitydistribution of the laser light in the resonator changes due todeterioration, for example, of an electrode in the resonator, therearises a problem that the bandwidth changes and expands in accordancewith such a change. This leads to non-stableness of a projectionexposure apparatus having such a laser, since in some cases it is notpossible to obtain a sufficient bandwidth necessary for suppressingchromatic aberration of a projection lens system or, in some cases, achange in the bandwidth of the laser light causes chromatic aberrationin the projection lens system.

SUMMARY OF THE INVENTION

It is accordingly a primary object of the present invention to providean improved exposure method and apparatus.

In accordance with a first aspect of the present invention, there isprovided a method of exposing a substrate with a band-narrowed laserbeam, wherein the laser beam is restricted to substantially compensatefor a change in the bandwidth of the laser beam.

In accordance with a second aspect of the present invention, there isprovided a method of exposing a substrate with a band-narrowed laserbeam, wherein, when the bandwidth of the laser beam becomes greater than0.005 nm, the laser beam is restricted to assure a bandwidth not greaterthan 0.005 nm, preferably not greater than 0.003 nm.

In accordance with a third aspect of the present invention, there isprovided a method of exposing a photosensitive substrate with aband-narrowed laser beam from an excimer laser, comprising: providing astop having an opening on a path of the laser beam; adjusting a size ofthe opening of the stop to substantially compensate for a change inbandwidth of the laser beam; and exposing after the adjustment thesubstrate with the laser beam.

In accordance with a fourth aspect of the present invention, there isprovided a method of manufacture of semiconductor devices by exposing aphotosensitive layer of a wafer to a band-narrowed laser beam from anexcimer laser to print a circuit pattern on the photosensitive layer,said method comprising: providing a stop having an opening, on a path ofthe laser beam; adjusting a size of the opening to substantiallycompensate for a change in bandwidth of the laser beam; and exposingafter the adjustment the wafer with the laser beam to print the circuitpattern on the wafer.

In accordance with a fifth aspect of the present invention, there isprovided a projection exposure apparatus, comprising: an excimer laserarranged to provide a band-narrowed laser beam; a reticle stage forsupporting a reticle; a wafer stage for supporting a wafer; anillumination system disposed between said laser and said reticle stage,for illuminating the reticle with the laser beam, said illuminationsystem having a stop member with an opening, for restricting the laserbeam, and an optical arrangement for expanding a diameter of the laserbeam passed through said opening and for providing a substantiallyuniform intensity distribution in the laser beam; a projection lenssystem disposed between said reticle stage and said wafer stage, forprojecting an image of a circuit pattern of the reticle on the wafer; anadjusting mechanism for adjusting a size of said opening of said stopmember; and a detector for detecting a change in sectional intensitydistribution of the laser beam; wherein said adjusting mechanism isoperable to adjust the size of said opening of said stop member inresponse to detection of the change in intensity distribution by saiddetector, to substantially compensate for a change in bandwidth of thelaser beam.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiment of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view, showing a general structure of a resonatorof an excimer laser.

FIG. 2 is a schematic and diagrammatic view, showing a first embodimentof the present invention.

FIG. 3A is a schematic view, showing an example of a detector formonitoring a wavelength distribution of laser light A.

FIG. 3B is a front view, schematically showing the relationship betweenthe laser light A and a slit 7.

FIG. 4 is a schematic and diagrammatic view, showing a second embodimentof the present invention.

FIG. 5 is a graph, illustrating wavelength spectrum in a section of alaser light.

FIG. 6 is a graph, illustrating dependence of the bandwidth of a laserlight upon the slit opening width.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a schematic view of a first embodiment of the presentinvention. Denoted in the drawing at 1 is a KrF excimer laser; at 2 is areflection type relief grating; at 3 is a prism having a function forexpanding the beam diameter so as to illuminate the grating 2 withuniform illuminance; at 4 and 4' are stops each having an aperture; at 5and 5' are windows of a discharging chamber 6; at 7 are dischargingelectrodes; and at 8 is an output mirror, all of which are similar tothose shown in FIG. 1. Reference character A in FIG. 2 denotes laserlight from the output mirror 8, having a center wavelength 248.4 nm anda bandwidth (halfwidth) greater than 0.005 nm. Reference character Bdenotes laser light having a narrowed bandwidth not greater than 0.003nm.

Denoted at 9 is a slit member having a variable-width aperture,changeable in the X-axis direction as illustrated; at 10 is anintroduction optical system for introducing light into a major part of asemiconductor exposure apparatus; at 11 is a total reflection mirror; at12 is an illumination optical system; at 13 is a reticle; at 14 is aprojection lens system; at 15 is a wafer; at 90 is a driver foractuating the slit 9; at 130 is a reticle stage; and at 150 is a waferstage. The lengthwise direction of the aperture of the slit 9 isparallel to the direction of lines (grating lines) of the grating 2 and,thus, it is perpendicular to the sheet of the drawing. The aperturewidth of the slit 9 is changeable in the X-axis direction (widthwisedirection), perpendicular to the lengthwise direction, by moving lightblocking plates of the slit member through the actuator 90 to change thespacing of the light blocking plates. Namely, the aperture width can beadjusted by the driving mechanism 90. Each light blocking plate of theslit 9 is made of a metal such as stainless steel or aluminum, forexample. FIG. 3A is an enlarged side view of the slit 9, and FIG. 3B isan enlarged front view thereof. Reference character AX in FIG. 3A or BBdenotes the optical axis of the illumination system (10, 11 and 12). Thebroken line with reference character A in FIG. 3B depicts the sectionalshape of the laser beam from the output mirror 8 which is rectangular.The widthwise direction of the rectangular shape coincides with the Xdirection (vertical) in this example as illustrated, and the lengthwisedirection thereof coincides with a direction (horizontal) perpendicularto the X direction.

Excimer gas sealingly contained in the discharging chamber 6 is excitedin response to application of a high voltage to the opposed dischargingelectrodes 7. The output mirror 8 cooperates with the grating 2 so as toprovide a resonator therebetween and, as a result, a band-narrowedexcimer laser light A can be emitted from the output mirror 8. Theband-narrowed laser light A produced from the excimer laser 1 goesthrough the variable-width slit 9. Here, under the influence of thevariable-width slit 9 as a stop, the spectral bandwidth of the laserlight A can be further narrowed without increasing the spatial coherencyof the laser light. Thus, an improved laser light B can be inputted intothe introduction optical system 10. The laser light passing through theintroduction optical system 10 is shaped and expanded by this opticalsystem 10 and, after being reflected by the total reflection mirror 11,it is inputted to the illumination optical system 12. In thisillumination optical system 12, the laser light is transformed intolight of uniform sectional intensity distribution in a known manner,such as disclosed in U.S. Pat. No. 4,974,191, and the transformed lightimpinges on the reticle 13. The projection lens system 14 projects animage of a circuit pattern of the reticle 13, illuminated with suchillumination light, upon the wafer 15 surface in a reduced scale. Inthis manner, a resist of the wafer 15 is exposed to the circuit patternimage, whereby a circuit pattern is printed on the wafer in a reducedscale.

Here, the function of the variable-width slit 9, namely, the influencethereof for narrowing the spectral bandwidth of the laser light withoutincreasing the spatial coherency thereof, will be explained in detail.As discussed with reference to equation (1), the wavelength with areflection angle θ_(B) of the grating 2 can be expressed by:

    λ=(2d/m)sinθ.sub.B

In FIG. 2, in the sectional plane of the laser light A just after beingemitted from the output mirror 8, X axis is laid on the direction (Xdirection) perpendicular to the direction of lines of the grating 2. Onthe other hand, it is assumed that, when a light ray is reflected in theresonator by the grating 2 at an angle θ_(B) (=θ₀), the position of thislight ray in the laser light A on that beam section is taken as anorigin for the X axis (i.e., coordinate x=0). If the angle θ_(B) changesfrom θ₀ to θ₀ +Δθ, then the coordinate x changes from 0 to Δx. Here, Δxand Δθ can be approximated as Δx=k·Δθ, wherein k is a proportionalconstant. If Δθ<<1, then a change Δλ in wavelength can be approximatedas follows: ##EQU1##

FIG. 5 shows a change in wavelength of a laser light emanating from anoutput mirror of a resonator on an occasion when a grating and a prismare used as the band narrowing means, with respect to a directioncorresponding to an axis perpendicular to the grating line and crossingthe path of the laser light. It is seen in the drawing that within thebeam the wavelength changes linearly, reflecting the dispersion by thegrating. Here, if the beam center is at a position x=0, for example, andthe position x=0 is taken as a reference, a shift Δλ of wavelength isgreater with an increase in the absolute value of the coordinate x. Fromthis, it is seen that, by disposing a slit 9 with a width variable inthe X direction (corresponding to the widthwise direction of the beamsection which is perpendicular to the grating line of the grating) afteran output mirror 8, as in the present embodiment, to thereby restrictthe width of the laser light A in the X direction, it is possible tosuppress the spectrum of laser light A to a value near that at x=0 and,therefore, it is possible to obtain a laser light B of very narrowedbandwidth. Since the variable-width slit 9 is provided outside theresonator of the laser 1, it does not strengthen relatively weak spatialcoherency which is an inherent property of the laser 1.

FIG. 6 shows the results when a band-narrowed laser light from a KrFexcimer laser is restricted by a slit with a varying aperture width, andillustrates the relationship between the aperture width (mm) of the slitand the bandwidth (halfwidth (pm: picometer)) of the laser light passedthrough the aperture. As seen in FIG. 6, the aperture diameter of thestop and the bandwidth (halfwidth) of the laser light are in asubstantially linear relationship.

The graphs of FIGS. 5 and 6 were prepared on the basis of results ofmeasurement made at a distance 2 m from the emission end of a KrFexcimer laser.

The present embodiment uses such a linear relationship between theaperture diameter of the stop and the bandwidth (halfwidth) of the laserlight, and the width of the aperture of the slit 9 is changed so as toprovide an adjusted bandwidth (halfwidth) of laser light B. As a result,even if the bandwidth of the laser light A changes and expands with timedue to deterioration of the electrode 6 of the laser 1, for example, bydetecting such change in the bandwidth and by narrowing the aperturewidth of the slit 9 through an appropriate quantity, it is possible tosupply laser light B of a predetermined and desired bandwidthconstantly. In this embodiment, the projection lens system 14 isprovided by a lens assembly made of a glass material of syntheticquartz. In order to suppress chromatic aberration of the projection lenssystem 14, in this embodiment the width of the slit 9 is adjusted sothat the bandwidth of the laser light B does not become greater than0.003 mm. It is to be noted here that, since an appropriate bandwidth oflaser light is determined in dependence upon the linewidth of a circuitpattern or the type of lens assembly used, the bandwidth of the laserlight B is not limited to a value not greater than 0.003 nm.Satisfactory results may be obtained with a bandwidth of about 0.005 nmor smaller.

A photodetector for detecting the bandwidth of the laser light A maycomprise any one of various types of photodetectors. In this embodiment,a detector such as shown in FIG. 3A is used to detect the bandwidth. InFIG. 3A, reference numeral 30 denotes a photodetector such as a PSD orthe like. The photodetector 30 may be moved so as to traverse the pathof the laser light A, such that while scanning the photodetector 30 withthe laser light A, the output of the photodetector 30 is monitored. Inthis manner, the intensity distribution of the laser light A in itswidthwise direction (X direction) in section can be detected. From theobtained intensity distribution and the inherent spatial wavelengthdistribution (FIG. 5) of the laser 1, the wavelength spectrum of thelaser light A can be determined and, from the wavelength spectrum, thebandwidth (halfwidth) can be detected.

On an occasion when only the expansion of the bandwidth of the laserlight A is to be detected, only a change in the intensity distributionof the laser light A from its initial state (which may be measuredbeforehand), may be detected. Thus, as an alternative, a glass plate orthe like may be inserted across the path of the laser light A, and achange in the intensity distribution can be detected by observing theilluminance distribution upon the glass plate.

In place of scanning the photodetector 30 with the laser light A, aone-dimensional or two-dimensional photoelectric converting elementarray may be inserted across the path of the laser light, so as todetect the intensity distribution of the laser light A.

The aperture width of the slit 9 may be adjusted by moving the slit 9manually or automatically, in accordance with the result of monitoringof the intensity distribution of the laser light A. When this is to bedone automatically, the photodetector 30 may be communicated with acontroller of the exposure apparatus through a signal line, so that anoutput signal from the photodetector may be processed by the controllerto determine the bandwidth and the quantity of movement of the slit 9 bythe driver 90 may be controlled accordingly.

Since the restriction of the laser light A by the slit 9 is necessarywhen the bandwidth of the laser light A is not narrowed as desired, itis possible to use such structure that the slit 9 is selectivelyinserted across the path of the laser light A. As an example, the slit 9may be mounted demountably.

The slit of this embodiment comprises two movable light blocking plates,but four light blocking plates may be used to define the aperture. Inthat case, two or all the four light blocking plates may be mademovable. As a further alternative, a single light blocking plate havingapertures of different diameters may be used, which light blocking platemay be moved so as to align an appropriate one of the apertures with thepath of the laser light, when desired.

In this embodiment, the laser light is restricted by the variable slit 9disposed outside the resonator of the laser 1 to accomplish narrowing ofthe bandwidth. However, such a variable slit may be provided within theresonator of the laser 1.

FIG. 4 is a schematic view of a major part of a second embodiment of thepresent invention. Like numerals as of FIG. 1 are assigned tocorresponding elements. The exposure apparatus of this embodiment isequipped with an intensity distribution monitor such as shown in FIG.3A.

Denoted in the drawing at 1 is a resonator of a KrF excimer laser (lightsource); at 2 is a grating which serves as a dispersing element for bandnarrowing; at 4 and 4' are aperture stops in the resonator; at 5 and 5'are windows of a discharging chamber 6; at 7 are opposed dischargingelectrodes; and at 8 is an output mirror. Outside the resonator 1 of thelaser 1, there are a variable-width slit 9 having an aperture variablein the x direction (FIG. 3B), an introduction optical system 10 forintroducing light into a major part of the semiconductor exposureapparatus, a total reflection mirror 11, an illumination optical system12, a reticle 13 for manufacture of semiconductor devices, a projectionlens system 14 and a wafer 15.

One feature of the FIG. 4 structure resides in use of a cylindricalexpander 16 as an optical system in the resonator for uniformlyilluminating the grating 2. With this structure, all the componentsother than the grating 2, which is disposed obliquely, can be disposedperpendicularly on the optical axis of the laser. Therefore, thedisposition of the device is simple. The cylindrical expander 16 isprovided by a cylindrical lens having its generating line disposedcoincident with the direction of lines of the grating 2.

The operation of this embodiment is similar to that shown in FIG. 2.That is, an excimer gas sealingly contained in the discharging chamber 6is excited in response to an application of a high voltage to theopposed discharging electrodes 7 and, as a result, band-narrowed excimerlaser light is emitted from the output mirror 8. The laser light Aproduced by the KrF excimer laser 1 goes through the aperture of thevariable-width slit 9. Here, under the influence of the variable-widthslit 9, the spectral bandwidth is further narrowed without enlarging thespatial coherency, whereby laser light B is inputted to the introductionoptical system 10. The laser light passing through the introductionoptical system 10, the mirror 11 and the illumination optical system 12,impinges on the reticle 13, such that the wafer 15 is exposed to animage of the circuit pattern of the reticle 13 by means of theprojection lens system 14.

In the preceding embodiments, the variable-width slit 9 is disposedbetween the band-narrowed excimer laser 1 and the introduction opticalsystem 10. However, the position of the variable-width slit 9 is notlimited thereto. If in an optical system after the introduction opticalsystem 10 there is a position which is optically conjugate with theposition shown in FIG. 2 or 3, the variable-width slit 9 may be disposedat such position, with substantially the same effect as that of thepreceding embodiment. Such a position may be on the path of light beforeit impinges on the reticle 13. Examples are a position within theintroduction optical system 10, a position between the introductionoptical system 10 and the illumination optical system 12 and a positionwithin the illumination optical system 12. In any case, the lengthwisedirection of the variable-width slit 9 is placed parallel to thedirection of the grating lines, when taken at the grating 2 position,and expansion control is made to the laser light A from the laser 1,with respect to the X direction of an angle of 90 degrees to thedirection of the grating line, as in the preceding embodiment. Theaperture width of the slit 9 may be held fixed during a certain periodand may be changed as required to adjust the bandwidth of the laserlight B. Preferably, the slit may be disposed at such portion of thepath at which the laser light is substantially parallel.

By means of a slit (stop) provided outside the resonator of the laser,the bandwidth of laser light can be effectively narrowed withoutincreasing the spatial coherency of the laser light itself. Accordingly,in a semiconductor exposure method and apparatus using a band-narrowedlaser as a light source, since the spatial coherency is notstrengthened, on one hand it is possible to suppress unwantedinterference such as speckle or the like. On the other hand, as anadvantageous result of narrowed spectral bandwidth, it is possible toreduce the effect of chromatic aberration of the projection lens systemand, thus, to enhance the resolution. Consequently, semiconductordevices of good quality can be manufactured stably.

While the foregoing embodiments have been explained with reference to anexposure apparatus having a KrF excimer laser as a light source, adifferent type excimer laser using a different excimer gas or adifferent type laser may be used as the light source. Further, theprojection lens system is not limited to a single-material lensassembly, and as an example a lens assembly made of different glassmaterials such as synthetic quartz and fluorite (CaF₂) may be used.

The present invention is not limited to an exposure apparatus ofprojection type. As an example, the present invention is applicable toan apparatus wherein deep ultraviolet laser light is focused on aworkpiece such as a wafer by means of a lens assembly to draw a patternthereon by the laser light. In that case, like the foregoingembodiments, the lens assembly may be made of a single glass materialcontaining synthetic quartz as a major component. Such advantageouseffects result from application of the present invention to a laser forproducing laser light of a wavelength not greater than 300 nm, withrespect to which a usable glass material is very limited.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. A method of exposing a photosensitive substrate with a band-narrowed laser beam from an excimer laser, said method comprising the steps of:detecting a change in bandwidth of the laser beam; changing the diameter of the laser beam to substantially compensate for the change in bandwidth of the laser beam; and exposing the substrate with the diameter-changed laser beam.
 2. A method according to claim 1, wherein said changing step comprises providing a stop having an aperture of adjustable size in the path of the laser beam.
 3. A method according to claim 2, wherein the excimer laser includes a resonator having a diffraction grating disposed therewithin, and wherein the size of the opening is adjusted by changing the width of the opening in a direction corresponding to a direction perpendicular to a grating line of the diffraction grating.
 4. A method according to claim 2, wherein the size of the opening is adjusted by changing the width of the opening that corresponds to a smaller dimension of a cross-section of the laser beam.
 5. A method according to claim 3 or 4, wherein a sectional intensity distribution of the laser beam is monitored to detect the change in the bandwidth.
 6. A method according to claim 2, wherein the excimer laser comprises a KrF excimer laser.
 7. A method according to claim 2, wherein the laser beam passing through the opening of the stop is directed to the substrate through a lens assembly, for exposure of the substrate.
 8. A method according to claim 7, wherein the size of the opening is adjusted so as to provide a laser beam of a bandwidth not greater than 0.005 nm.
 9. A method according to claim 8, wherein the size of the aperture is adjusted so as to provide a laser beam of a bandwidth not greater than 0.003 nm.
 10. A method according to claim 7, wherein the lens assembly is made substantially of a single glass material.
 11. A method according to claim 10, wherein the glass material includes synthetic quartz as a major component.
 12. A method of manufacturing semiconductor devices by exposing a photosensitive layer of a wafer to a band-narrowed laser beam from an excimer laser to print a circuit pattern on the photosensitive layer, said method comprising the steps of:detecting a change in bandwidth of the laser beam; changing the diameter of the laser beam to substantially compensate for the change in bandwidth of the laser beam; and exposing the wafer with the diameter-changed laser beam to print the circuit pattern on the wafer.
 13. A method according to claim 12, wherein said changing step comprises providing a stop having an aperture of adjustable size in the path of the laser beam.
 14. A method according to claim 13, wherein the circuit pattern is illuminated with the laser beam passing through the opening of the stop, and wherein an image of the circuit pattern is projected on the wafer by a projection lens system, for exposure of the wafer.
 15. A method according to claim 14, wherein the excimer laser includes a resonator having a diffraction grating disposed therewithin, and wherein the size of the opening is adjusted by changing the width of the opening in a direction corresponding to a direction perpendicular to a grating line of the diffraction grating.
 16. A method according to claim 14, wherein the size of the opening is adjusted by changing the width of the opening that corresponds to a smaller dimension of a cross-section of the laser beam.
 17. A method according to claim 15 or 16, wherein a sectional intensity distribution of the laser beam is monitored to detect the change in the bandwidth.
 18. A method according to claim 14, wherein the lens assembly is made substantially of a single glass material.
 19. A method according to claim 18, wherein the glass material includes synthetic quartz as a major component.
 20. A method according to claim 19, wherein the size of the opening is adjusted so as to provide a laser beam of a bandwidth not greater than 0.005 nm.
 21. A method according to claim 20, wherein the size of the aperture is adjusted so as to provide a laser beam of a bandwidth not greater than 0.003 nm.
 22. A method of exposing a photosensitive substrate with a band-narrowed laser beam, said method comprising the steps of:detecting a change in bandwidth of the laser beam; changing the diameter of the laser beam to substantially compensate for the change in bandwidth of the laser beam; and exposing the substrate with the diameter-changed laser beam.
 23. A method according to claim 22, wherein said changing step comprises providing a stop having an aperture of adjustable size in a path of the laser beam.
 24. A method according to claim 22, wherein said exposing step comprises focusing the laser beam onto the substrate by a projection lens.
 25. A device manufacturing method including a step of exposing a photosensitive substrate with a band-narrowed laser beam to print a device pattern on the substrate, said method comprising the steps of:detecting a change in bandwidth of the laser beam; changing the diameter of the laser beam to substantially compensate for the change in bandwidth of the laser beam; and exposing the substrate with the diameter-changed laser beam.
 26. A method according to claim 25, wherein said changing step comprises providing a stop having an aperture of adjustable size in a path of the laser beam.
 27. A method according to claim 25, wherein said exposing step comprises focusing the laser beam onto the substrate by a projection lens.
 28. A method of exposing a photosensitive substrate with a band-narrowed laser beam, said method comprising the steps of:providing a variable aperture in a path of the laser beam; detecting a change in bandwidth of the laser beam; and substantially compensating for the detected change in bandwidth of the laser beam by changing the width of the aperture.
 29. A method of manufacturing a device by exposing a photosensitive substrate with a band-narrowed laser beam to print a device pattern on the substrate, said method comprising the steps of:providing a variable aperture in a path of the laser beam; detecting a change in bandwidth of the laser beam; and substantially compensating for the detected change in bandwidth of the laser beam by changing the width of the aperture. 